/* Hash Tables Implementation.
 *
 * This file implements in memory hash tables with insert/del/replace/find/
 * get-random-element operations. Hash tables will auto resize if needed
 * tables of power of two in size are used, collisions are handled by
 * chaining. See the source code for more information... :)
 *
 * Copyright (c) 2006-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include "fmacros.h"

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <stdarg.h>
#include <limits.h>
#include <sys/time.h>

#include "dict.h"
#include "zmalloc.h"
#include "redisassert.h"

/* Using dictEnableResize() / dictDisableResize() we make possible to
 * enable/disable resizing of the hash table as needed. This is very important
 * for Redis, as we use copy-on-write and don't want to move too much memory
 * around when there is a child performing saving operations.
 *
 * Note that even when dict_can_resize is set to 0, not all resizes are
 * prevented: a hash table is still allowed to grow if the ratio between
 * the number of elements and the buckets > dict_force_resize_ratio. */
static int dict_can_resize = 1;
static unsigned int dict_force_resize_ratio = 5;

/* -------------------------- private prototypes ---------------------------- */

static int _dictExpandIfNeeded(dict *ht);
static unsigned long _dictNextPower(unsigned long size);
static long _dictKeyIndex(dict *ht, const void *key, uint64_t hash, dictEntry **existing);
static int _dictInit(dict *ht, dictType *type, void *privDataPtr);

/* -------------------------- hash functions -------------------------------- */

static uint8_t dict_hash_function_seed[16];

void dictSetHashFunctionSeed(uint8_t *seed) {
    memcpy(dict_hash_function_seed,seed,sizeof(dict_hash_function_seed));
}

uint8_t *dictGetHashFunctionSeed(void) {
    return dict_hash_function_seed;
}

/* The default hashing function uses SipHash implementation
 * in siphash.c. */

uint64_t siphash(const uint8_t *in, const size_t inlen, const uint8_t *k);
uint64_t siphash_nocase(const uint8_t *in, const size_t inlen, const uint8_t *k);

uint64_t dictGenHashFunction(const void *key, int len) {
    return siphash(key,len,dict_hash_function_seed);
}

uint64_t dictGenCaseHashFunction(const unsigned char *buf, int len) {
    return siphash_nocase(buf,len,dict_hash_function_seed);
}

/* ----------------------------- API implementation ------------------------- */

/* Reset a hash table already initialized with ht_init().
 * NOTE: This function should only be called by ht_destroy(). */
static void _dictReset(dictht *ht)
{
    ht->table = NULL;
    ht->size = 0;
    ht->sizemask = 0;
    ht->used = 0;
}

// 创建一个新的字典
dict *dictCreate(dictType *type,
        void *privDataPtr)
{
    // 申请内存空间
    dict *d = zmalloc(sizeof(*d));
    // 初始化字典
    _dictInit(d,type,privDataPtr);
    return d;
}

/* Initialize the hash table */
int _dictInit(dict *d, dictType *type,
        void *privDataPtr)
{
    // 初始化两个哈希表的各项属性值，但暂时还不分配内存给哈希表数组
    //重置两个哈希表
    _dictReset(&d->ht[0]);
    _dictReset(&d->ht[1]);
    // 设置类型特定函数
    d->type = type;
    // 设置私有数据
    d->privdata = privDataPtr;
    // 设置哈希表 rehash 状态。没rehash=-1，rehash中=0
    d->rehashidx = -1;
    d->pauserehash = 0;
    return DICT_OK;// 初始化成功
}

/*
 * 缩小给定字典
 * 让它的已用节点数和字典大小之间的比率接近 1:1
 * 返回 DICT_ERR 表示字典已经在 rehash ，或者 dict_can_resize 为假。
 * 成功创建体积更小的 ht[1] ，可以开始 resize 时，返回 DICT_OK。
 */
int dictResize(dict *d)
{
    unsigned long minimal;//新表所需结点的最小数量
    // 不能在关闭 rehash 或者正在 rehash 的时候调用
    if (!dict_can_resize || dictIsRehashing(d)) return DICT_ERR;
    minimal = d->ht[0].used;    // 计算让比率接近 1：1 所需要的最少节点数量（ht[0].used赋值过来）
    if (minimal < DICT_HT_INITIAL_SIZE)// 如果used太小，则调整为默认最小值
        minimal = DICT_HT_INITIAL_SIZE;
    return dictExpand(d, minimal);// 调整后开始调用“扩展或者创建一个新的哈希表”方法
}

/*
 * 扩展或者创建一个新的哈希表
 * 创建一个新的哈希表，并根据字典的情况，选择以下其中一个动作来进行：
 * 1) 如果字典的 0 号哈希表为空，那么将新哈希表设置为 0 号哈希表
 * 2) 如果字典的 0 号哈希表非空，那么将新哈希表设置为 1 号哈希表，并打开字典的 rehash 标识，使得程序可以开始对字典进行 rehash
 * size 参数不够大，或者 rehash 已经在进行时，返回 DICT_ERR 。
 * 成功创建 0 号哈希表，或者 1 号哈希表时，返回 DICT_OK 。
 */
int _dictExpand(dict *d, unsigned long size, int* malloc_failed)
{
    if (malloc_failed) *malloc_failed = 0;

    // 不能在字典正在 rehash 时进行扩展表操作，size 的值也不能小于 0 号哈希表的当前已使用节
    if (dictIsRehashing(d) || d->ht[0].used > size)
        return DICT_ERR;

    dictht n; /* the new hash table */
    // 根据 size 参数，计算新哈希表的大小
    unsigned long realsize = _dictNextPower(size);

    if (realsize == d->ht[0].size) return DICT_ERR;

    // 确定新哈希表大小
    n.size = realsize;
    n.sizemask = realsize-1;
    // 为1号哈希表分配内存空间
    if (malloc_failed) {
        // 分配失败，用ztrycalloc尝试分配空间
        n.table = ztrycalloc(realsize*sizeof(dictEntry*));
        *malloc_failed = n.table == NULL;
        if (*malloc_failed)
            return DICT_ERR;
    } else // 用zcalloc普通方式分配空间
        n.table = zcalloc(realsize*sizeof(dictEntry*));
    // 初始化节点数量为0
    n.used = 0;

   // 如果ht[0]是空的，那说明是初始化，直接初始化table即可，不用初始化ht[1]
    if (d->ht[0].table == NULL) {
        d->ht[0] = n;
        return DICT_OK;
    }

    // 来到这步说明是开始rehash，初始化ht[1]，然后初始化rehashidx=0
    d->ht[1] = n;
    d->rehashidx = 0;
    return DICT_OK;
}

// 扩展或者创建一个新的哈希表的重载方法
int dictExpand(dict *d, unsigned long size) {
    return _dictExpand(d, size, NULL);
}

/* return DICT_ERR if expand failed due to memory allocation failure */
int dictTryExpand(dict *d, unsigned long size) {
    int malloc_failed;
    _dictExpand(d, size, &malloc_failed);
    return malloc_failed? DICT_ERR : DICT_OK;
}

/*
 * 执行 N 步渐进式 rehash 。
 * 返回 1 表示仍有键需要从 0 号哈希表移动到 1 号哈希表，
 * 返回 0 则表示所有键都已经迁移完毕。
 * 注意，每步 rehash 都是以一个哈希表索引（桶）作为单位的，
 * 一个桶里可能会有多个节点，
 * 被 rehash 的桶里的所有节点都会被移动到新哈希表。
 */
int dictRehash(dict *d, int n) {
    // 这里的n代表一共要迁移多少个dictEntry
    // 只可以在 rehash 进行中时执行
    int empty_visits = n*10; /* Max number of empty buckets to visit. */
    if (!dictIsRehashing(d)) return 0;// 如果不是正在rehash中，就中断
    // 进行 N 步迁移
    while(n-- && d->ht[0].used != 0) {
        dictEntry *de, *nextde;

        /* Note that rehashidx can't overflow as we are sure there are more
         * elements because ht[0].used != 0 */
        assert(d->ht[0].size > (unsigned long)d->rehashidx);
        // 找到第一个非空索引的下标
        while(d->ht[0].table[d->rehashidx] == NULL) {
0.
            if (--empty_visits == 0) return 1;
        }
        // 指向第一个非空索引的链表表头节点
        de = d->ht[0].table[d->rehashidx];
        // 将链表中的所有节点迁移到新哈希表（重新计算位置，在新表上可能就不是在一条链上了）
        while(de) {
            uint64_t h;
            // 保存下个节点的指针
            nextde = de->next;
            //计算结点插入的位置索引=key的哈希值 & sizemask
            h = dictHashKey(d, de->key) & d->ht[1].sizemask;
            // 插入节点到新哈希表,而且是插入到表头（每次插入都是插到链表的表头）
            de->next = d->ht[1].table[h];
            d->ht[1].table[h] = de;
            // 更新计数器，0数量-1,1数量+1
            d->ht[0].used--;
            d->ht[1].used++;
            de = nextde;// 继续处理下一个节点
        }
        // 将刚迁移完的哈希表索引的指针设为空
        d->ht[0].table[d->rehashidx] = NULL;
        d->rehashidx++;
    }

    // 检查是否重新hash整个ht[0]，并做后置处理
    if (d->ht[0].used == 0) {
        zfree(d->ht[0].table);// 释放ht[0]空间
        d->ht[0] = d->ht[1];// 将ht[1]替换ht[0]
        _dictReset(&d->ht[1]);// 清空重置ht[1]
        d->rehashidx = -1;//重置rehashidx = -1
        return 0;
    }

    /* More to rehash... */
    return 1;
}

/*
 * 返回以毫秒为单位的 UNIX 时间戳
 */
long long timeInMilliseconds(void) {
    struct timeval tv;
    // 获得当前时间的精准值
    gettimeofday(&tv,NULL);
    // 强制类型转换，避免溢出
    return (((long long)tv.tv_sec)*1000)+(tv.tv_usec/1000);
}

/*
 * 在给定毫秒数内，以 100 步为单位, 对字典进行 rehash.也就是说每次对100个dictEntry进行hash.
 */
int dictRehashMilliseconds(dict *d, int ms) {
    if (d->pauserehash > 0) return 0;

    long long start = timeInMilliseconds();
    int rehashes = 0;//这一次迁移完成的dictntry个数
    // 每次迁移100个。如果时间已过，跳出
    while(dictRehash(d,100)) {
        rehashes += 100;
        if (timeInMilliseconds()-start > ms) break;
    }
    return rehashes;// 返回迁移完成个数
}

/*
 * 在字典不存在安全迭代器的情况下，对字典进行单步 rehash 。
 *
 * 字典有安全迭代器的情况下不能进行 rehash ，
 * 因为两种不同的迭代和修改操作可能会弄乱字典。
 * 这个函数被多个通用的查找、更新操作调用，
 * 它可以让字典在被使用的同时进行 rehash 。
 */
static void _dictRehashStep(dict *d) {
    // 要求该字典上不存在安全迭代器
    if (d->pauserehash == 0) dictRehash(d,1);
}

/*
 * 先是调用dictAddRaw添加键，然后调用dictSetVal添加键的值，这样才算是添加了键值对
 * 尝试将给定键值对添加到字典中
 * 只有给定键 key 不存在于字典时，添加操作才会成功
 * 添加成功返回 DICT_OK , 失败返回 DICT_ERR
 * 最坏 T = O(N),平摊 O(1)
 */
int dictAdd(dict *d, void *key, void *val)
{
    // 尝试添加键到字典，并返回包含了这个键的新哈希节点  O(n)
    dictEntry *entry = dictAddRaw(d,key,NULL);

    if (!entry) return DICT_ERR;// 键已存在，添加失败
    dictSetVal(d, entry, val);// 键不存在，设置节点的值
    return DICT_OK;
}

/*
 * 尝试将键插入到字典中
 *
 * 如果键已经在字典存在，那么返回 NULL
 *
 * 如果键不存在，那么程序创建新的哈希节点，
 * 将节点和键关联，并插入到字典，然后返回节点本身。
 */
dictEntry *dictAddRaw(dict *d, void *key, dictEntry **existing)
{
    long index;// 节点索引
    dictEntry *entry;//节点
    dictht *ht;// 哈希表
    // 如果条件允许的话，进行单步 rehash
    // 如果需要rehashing~,那么我们进行rehash,注意,这里是单步rehash
    if (dictIsRehashing(d)) _dictRehashStep(d);

    // 计算键在哈希表中的索引值
    // 如果值为 -1 ，那么表示键已经存在
    if ((index = _dictKeyIndex(d, key, dictHashKey(d,key), existing)) == -1)
        return NULL;

    // 如果字典正在 rehash ，那么将新键添加到 1 号哈希表，否则，将新键添加到 0 号哈希表
    ht = dictIsRehashing(d) ? &d->ht[1] : &d->ht[0];
    entry = zmalloc(sizeof(*entry));// 为新节点分配空间
    // 将新节点插入到链表表头,因为键不存在，所以index肯定是表头
    entry->next = ht->table[index];
    ht->table[index] = entry;
    ht->used++;// 更新哈希表已使用节点数量

    // 设置新节点的键
    dictSetKey(d, entry, key);
    return entry;
}

/*
 * 将给定的键值对添加到字典中，如果键已经存在，那么删除旧有的键值对。
 *
 * 如果键值对为全新添加，那么返回 1 。
 * 如果键值对是通过对原有的键值对更新得来的，那么返回 0 。
 *
 * T = O(N)
 */
int dictReplace(dict *d, void *key, void *val)
{
    dictEntry *entry, *existing, auxentry;

    // 尝试直接将键值对添加到字典 如果键 key 不存在的话，添加会成功
    entry = dictAddRaw(d,key,&existing);
    if (entry) {
        dictSetVal(d, entry, val);
        return 1;
    }

    // 设置新值，释放旧值
    auxentry = *existing;
    dictSetVal(d, existing, val);
    dictFreeVal(d, &auxentry);
    return 0;
}

/**
* 找到对应的key，如果就给他加进去并返回
**/
dictEntry *dictAddOrFind(dict *d, void *key) {
    dictEntry *entry, *existing;
    entry = dictAddRaw(d,key,&existing);// 先看看能不能添加成功，成功就返回信息，失败就说明已存在返回之前的
    return entry ? entry : existing;
}

/*
 * 查找并删除包含给定键的节点
 *
 * 参数 nofree 决定是否调用键和值的释放函数
 * 0 表示调用，1 表示不调用
 *
 * 找到并成功删除返回 DICT_OK ，没找到则返回 DICT_ERR
 *
 * T = O(1)
 */
static dictEntry *dictGenericDelete(dict *d, const void *key, int nofree) {
    uint64_t h, idx;
    dictEntry *he, *prevHe;
    int table;
    // 字典表为空，直接删除掉
    if (d->ht[0].used == 0 && d->ht[1].used == 0) return NULL;
    //在 rehash期间 可以进行单步 rehash
    if (dictIsRehashing(d)) _dictRehashStep(d);
    h = dictHashKey(d, key);// 计算键的哈希值
    // 遍历处理ht[0]，ht[1]。如果0中有就删除，没有就去1找，找到就删除
    for (table = 0; table <= 1; table++) {
        idx = h & d->ht[table].sizemask;
        he = d->ht[table].table[idx];
        prevHe = NULL;
        while(he) {// 拉链法根据hash的位置找到对应的数据
            if (key==he->key || dictCompareKeys(d, key, he->key)) {
                // 找到对应的值，把它从链表中删除
                if (prevHe)
                    prevHe->next = he->next;
                else
                    d->ht[table].table[idx] = he->next;
                // 然后看情况要不要释放空间
                if (!nofree) {
                    dictFreeKey(d, he);
                    dictFreeVal(d, he);
                    zfree(he);
                }
                // 然后把已占用的数量-1
                d->ht[table].used--;
                return he;
            }
            // 到这步，说明找不到，这时候找下一个节点看一下（拉链法解决hash冲突）
            prevHe = he;
            he = he->next;
        }
        // 到这步，说明ht[0]找不到，并且没有rehashing中，说明
        if (!dictIsRehashing(d)) break;
    }
    return NULL; /* not found */
}

/**
* 删除对应的键
**/
int dictDelete(dict *ht, const void *key) {
    return dictGenericDelete(ht,key,0) ? DICT_OK : DICT_ERR;
}

/* Remove an element from the table, but without actually releasing
 * the key, value and dictionary entry. The dictionary entry is returned
 * if the element was found (and unlinked from the table), and the user
 * should later call `dictFreeUnlinkedEntry()` with it in order to release it.
 * Otherwise if the key is not found, NULL is returned.
 *
 * This function is useful when we want to remove something from the hash
 * table but want to use its value before actually deleting the entry.
 * Without this function the pattern would require two lookups:
 *
 *  entry = dictFind(...);
 *  // Do something with entry
 *  dictDelete(dictionary,entry);
 *
 * Thanks to this function it is possible to avoid this, and use
 * instead:
 *
 * entry = dictUnlink(dictionary,entry);
 * // Do something with entry
 * dictFreeUnlinkedEntry(entry); // <- This does not need to lookup again.
 */
dictEntry *dictUnlink(dict *ht, const void *key) {
    return dictGenericDelete(ht,key,1);
}

/* You need to call this function to really free the entry after a call
 * to dictUnlink(). It's safe to call this function with 'he' = NULL. */
void dictFreeUnlinkedEntry(dict *d, dictEntry *he) {
    if (he == NULL) return;
    dictFreeKey(d, he);
    dictFreeVal(d, he);
    zfree(he);
}

/*
 * 删除哈希表上的所有节点，并重置哈希表的各项属性
 * T = O(N)
 */
int _dictClear(dict *d, dictht *ht, void(callback)(void *)) {
    unsigned long i;
    // 遍历所有节点，然后进行释放
    for (i = 0; i < ht->size && ht->used > 0; i++) {
        dictEntry *he, *nextHe;

        if (callback && (i & 65535) == 0) callback(d->privdata);

        if ((he = ht->table[i]) == NULL) continue;
        // 在链表上一个个节点释放过去
        while(he) {
            nextHe = he->next;
            dictFreeKey(d, he);
            dictFreeVal(d, he);
            zfree(he);
            ht->used--;
            he = nextHe;
        }
    }
    // 然后释放哈希表本身占用空间
    zfree(ht->table);
    // 重置哈希表属性
    _dictReset(ht);
    return DICT_OK; /* never fails */
}

/*
 * 删除并释放整个字典
 * T = O(N)
 */
void dictRelease(dict *d)
{
    // 清除
    _dictClear(d,&d->ht[0],NULL);
    _dictClear(d,&d->ht[1],NULL);
    zfree(d);// 清除整个最外层的结构
}

/*
 * 返回字典中包含键 key 的节点
 * 找到返回节点，找不到返回 NULL
 * T = O(1)
 */
dictEntry *dictFind(dict *d, const void *key)
{
    dictEntry *he;
    uint64_t h, idx, table;
    // 字典为空，直接返回找不到
    if (dictSize(d) == 0) return NULL; /* dict is empty */
    // 在rehash中，进行单步rehash
    if (dictIsRehashing(d)) _dictRehashStep(d);
    h = dictHashKey(d, key);// 计算hash值
    // 先去ht[0]找，找不到去ht[1]找
    for (table = 0; table <= 1; table++) {
        idx = h & d->ht[table].sizemask;
        he = d->ht[table].table[idx];
        while(he) {// 拉链法解决的冲突
            if (key==he->key || dictCompareKeys(d, key, he->key))
                return he;
            he = he->next;
        }
        // 到这步说明ht[0]找不到，并且不处于rehash中，直接中断返回找不到
        if (!dictIsRehashing(d)) return NULL;
    }
    return NULL;
}

/*
 * 获取包含给定键的节点的值
 *
 * 如果节点不为空，返回节点的值
 * 否则返回 NULL
 * T = O(1)
 */
void *dictFetchValue(dict *d, const void *key) {
    dictEntry *he;
    he = dictFind(d,key);//根据键在字典中找到该结点
    return he ? dictGetVal(he) : NULL;//结点不空的话，返回结点的值
}

/* A fingerprint is a 64 bit number that represents the state of the dictionary
 * at a given time, it's just a few dict properties xored together.
 * When an unsafe iterator is initialized, we get the dict fingerprint, and check
 * the fingerprint again when the iterator is released.
 * If the two fingerprints are different it means that the user of the iterator
 * performed forbidden operations against the dictionary while iterating. */
long long dictFingerprint(dict *d) {
    long long integers[6], hash = 0;
    int j;

    integers[0] = (long) d->ht[0].table;
    integers[1] = d->ht[0].size;
    integers[2] = d->ht[0].used;
    integers[3] = (long) d->ht[1].table;
    integers[4] = d->ht[1].size;
    integers[5] = d->ht[1].used;

    /* We hash N integers by summing every successive integer with the integer
     * hashing of the previous sum. Basically:
     *
     * Result = hash(hash(hash(int1)+int2)+int3) ...
     *
     * This way the same set of integers in a different order will (likely) hash
     * to a different number. */
    for (j = 0; j < 6; j++) {
        hash += integers[j];
        /* For the hashing step we use Tomas Wang's 64 bit integer hash. */
        hash = (~hash) + (hash << 21); // hash = (hash << 21) - hash - 1;
        hash = hash ^ (hash >> 24);
        hash = (hash + (hash << 3)) + (hash << 8); // hash * 265
        hash = hash ^ (hash >> 14);
        hash = (hash + (hash << 2)) + (hash << 4); // hash * 21
        hash = hash ^ (hash >> 28);
        hash = hash + (hash << 31);
    }
    return hash;
}

/*
 * 创建并返回给定字典的不安全迭代器
 *
 * T = O(1)
 */
dictIterator *dictGetIterator(dict *d)
{
    dictIterator *iter = zmalloc(sizeof(*iter));

    iter->d = d;
    iter->table = 0;
    iter->index = -1;
    iter->safe = 0;
    iter->entry = NULL;
    iter->nextEntry = NULL;
    return iter;
}

/*
 * 创建并返回给定节点的安全迭代器
 *
 * T = O(1)
 */
dictIterator *dictGetSafeIterator(dict *d) {
    dictIterator *i = dictGetIterator(d);

    i->safe = 1;// 设置安全迭代器标识，设置了安全迭代标识之后，就不能再进行rehash,因为安全迭代过程中要确保数据不重复。不能一边迭代一边增删元素
    return i;
}

dictEntry *dictNext(dictIterator *iter)
{
    while (1) {
        if (iter->entry == NULL) {
            dictht *ht = &iter->d->ht[iter->table];
            if (iter->index == -1 && iter->table == 0) {
                if (iter->safe)
                    dictPauseRehashing(iter->d);
                else
                    iter->fingerprint = dictFingerprint(iter->d);
            }
            iter->index++;
            if (iter->index >= (long) ht->size) {
                if (dictIsRehashing(iter->d) && iter->table == 0) {
                    iter->table++;
                    iter->index = 0;
                    ht = &iter->d->ht[1];
                } else {
                    break;
                }
            }
            iter->entry = ht->table[iter->index];
        } else {
            iter->entry = iter->nextEntry;
        }
        if (iter->entry) {
            /* We need to save the 'next' here, the iterator user
             * may delete the entry we are returning. */
            iter->nextEntry = iter->entry->next;
            return iter->entry;
        }
    }
    return NULL;
}

void dictReleaseIterator(dictIterator *iter)
{
    if (!(iter->index == -1 && iter->table == 0)) {
        if (iter->safe)
            dictResumeRehashing(iter->d);
        else
            assert(iter->fingerprint == dictFingerprint(iter->d));
    }
    zfree(iter);
}

/* Return a random entry from the hash table. Useful to
 * implement randomized algorithms */
dictEntry *dictGetRandomKey(dict *d)
{
    dictEntry *he, *orighe;
    unsigned long h;
    int listlen, listele;

    if (dictSize(d) == 0) return NULL;
    if (dictIsRehashing(d)) _dictRehashStep(d);
    if (dictIsRehashing(d)) {
        do {
            /* We are sure there are no elements in indexes from 0
             * to rehashidx-1 */
            h = d->rehashidx + (randomULong() % (dictSlots(d) - d->rehashidx));
            he = (h >= d->ht[0].size) ? d->ht[1].table[h - d->ht[0].size] :
                                      d->ht[0].table[h];
        } while(he == NULL);
    } else {
        do {
            h = randomULong() & d->ht[0].sizemask;
            he = d->ht[0].table[h];
        } while(he == NULL);
    }

    /* Now we found a non empty bucket, but it is a linked
     * list and we need to get a random element from the list.
     * The only sane way to do so is counting the elements and
     * select a random index. */
    listlen = 0;
    orighe = he;
    while(he) {
        he = he->next;
        listlen++;
    }
    listele = random() % listlen;
    he = orighe;
    while(listele--) he = he->next;
    return he;
}

/* This function samples the dictionary to return a few keys from random
 * locations.
 *
 * It does not guarantee to return all the keys specified in 'count', nor
 * it does guarantee to return non-duplicated elements, however it will make
 * some effort to do both things.
 *
 * Returned pointers to hash table entries are stored into 'des' that
 * points to an array of dictEntry pointers. The array must have room for
 * at least 'count' elements, that is the argument we pass to the function
 * to tell how many random elements we need.
 *
 * The function returns the number of items stored into 'des', that may
 * be less than 'count' if the hash table has less than 'count' elements
 * inside, or if not enough elements were found in a reasonable amount of
 * steps.
 *
 * Note that this function is not suitable when you need a good distribution
 * of the returned items, but only when you need to "sample" a given number
 * of continuous elements to run some kind of algorithm or to produce
 * statistics. However the function is much faster than dictGetRandomKey()
 * at producing N elements. */
unsigned int dictGetSomeKeys(dict *d, dictEntry **des, unsigned int count) {
    unsigned long j; /* internal hash table id, 0 or 1. */
    unsigned long tables; /* 1 or 2 tables? */
    unsigned long stored = 0, maxsizemask;
    unsigned long maxsteps;

    if (dictSize(d) < count) count = dictSize(d);
    maxsteps = count*10;

    /* Try to do a rehashing work proportional to 'count'. */
    for (j = 0; j < count; j++) {
        if (dictIsRehashing(d))
            _dictRehashStep(d);
        else
            break;
    }

    tables = dictIsRehashing(d) ? 2 : 1;
    maxsizemask = d->ht[0].sizemask;
    if (tables > 1 && maxsizemask < d->ht[1].sizemask)
        maxsizemask = d->ht[1].sizemask;

    /* Pick a random point inside the larger table. */
    unsigned long i = randomULong() & maxsizemask;
    unsigned long emptylen = 0; /* Continuous empty entries so far. */
    while(stored < count && maxsteps--) {
        for (j = 0; j < tables; j++) {
            /* Invariant of the dict.c rehashing: up to the indexes already
             * visited in ht[0] during the rehashing, there are no populated
             * buckets, so we can skip ht[0] for indexes between 0 and idx-1. */
            if (tables == 2 && j == 0 && i < (unsigned long) d->rehashidx) {
                /* Moreover, if we are currently out of range in the second
                 * table, there will be no elements in both tables up to
                 * the current rehashing index, so we jump if possible.
                 * (this happens when going from big to small table). */
                if (i >= d->ht[1].size)
                    i = d->rehashidx;
                else
                    continue;
            }
            if (i >= d->ht[j].size) continue; /* Out of range for this table. */
            dictEntry *he = d->ht[j].table[i];

            /* Count contiguous empty buckets, and jump to other
             * locations if they reach 'count' (with a minimum of 5). */
            if (he == NULL) {
                emptylen++;
                if (emptylen >= 5 && emptylen > count) {
                    i = randomULong() & maxsizemask;
                    emptylen = 0;
                }
            } else {
                emptylen = 0;
                while (he) {
                    /* Collect all the elements of the buckets found non
                     * empty while iterating. */
                    *des = he;
                    des++;
                    he = he->next;
                    stored++;
                    if (stored == count) return stored;
                }
            }
        }
        i = (i+1) & maxsizemask;
    }
    return stored;
}

/* This is like dictGetRandomKey() from the POV of the API, but will do more
 * work to ensure a better distribution of the returned element.
 *
 * This function improves the distribution because the dictGetRandomKey()
 * problem is that it selects a random bucket, then it selects a random
 * element from the chain in the bucket. However elements being in different
 * chain lengths will have different probabilities of being reported. With
 * this function instead what we do is to consider a "linear" range of the table
 * that may be constituted of N buckets with chains of different lengths
 * appearing one after the other. Then we report a random element in the range.
 * In this way we smooth away the problem of different chain lengths. */
#define GETFAIR_NUM_ENTRIES 15
dictEntry *dictGetFairRandomKey(dict *d) {
    dictEntry *entries[GETFAIR_NUM_ENTRIES];
    unsigned int count = dictGetSomeKeys(d,entries,GETFAIR_NUM_ENTRIES);
    /* Note that dictGetSomeKeys() may return zero elements in an unlucky
     * run() even if there are actually elements inside the hash table. So
     * when we get zero, we call the true dictGetRandomKey() that will always
     * yield the element if the hash table has at least one. */
    if (count == 0) return dictGetRandomKey(d);
    unsigned int idx = rand() % count;
    return entries[idx];
}

/* Function to reverse bits. Algorithm from:
 * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */
static unsigned long rev(unsigned long v) {
    unsigned long s = CHAR_BIT * sizeof(v); // bit size; must be power of 2
    unsigned long mask = ~0UL;
    while ((s >>= 1) > 0) {
        mask ^= (mask << s);
        v = ((v >> s) & mask) | ((v << s) & ~mask);
    }
    return v;
}

/* dictScan() is used to iterate over the elements of a dictionary.
 *
 * Iterating works the following way:
 *
 * 1) Initially you call the function using a cursor (v) value of 0.
 * 2) The function performs one step of the iteration, and returns the
 *    new cursor value you must use in the next call.
 * 3) When the returned cursor is 0, the iteration is complete.
 *
 * The function guarantees all elements present in the
 * dictionary get returned between the start and end of the iteration.
 * However it is possible some elements get returned multiple times.
 *
 * For every element returned, the callback argument 'fn' is
 * called with 'privdata' as first argument and the dictionary entry
 * 'de' as second argument.
 *
 * HOW IT WORKS.
 *
 * The iteration algorithm was designed by Pieter Noordhuis.
 * The main idea is to increment a cursor starting from the higher order
 * bits. That is, instead of incrementing the cursor normally, the bits
 * of the cursor are reversed, then the cursor is incremented, and finally
 * the bits are reversed again.
 *
 * This strategy is needed because the hash table may be resized between
 * iteration calls.
 *
 * dict.c hash tables are always power of two in size, and they
 * use chaining, so the position of an element in a given table is given
 * by computing the bitwise AND between Hash(key) and SIZE-1
 * (where SIZE-1 is always the mask that is equivalent to taking the rest
 *  of the division between the Hash of the key and SIZE).
 *
 * For example if the current hash table size is 16, the mask is
 * (in binary) 1111. The position of a key in the hash table will always be
 * the last four bits of the hash output, and so forth.
 *
 * WHAT HAPPENS IF THE TABLE CHANGES IN SIZE?
 *
 * If the hash table grows, elements can go anywhere in one multiple of
 * the old bucket: for example let's say we already iterated with
 * a 4 bit cursor 1100 (the mask is 1111 because hash table size = 16).
 *
 * If the hash table will be resized to 64 elements, then the new mask will
 * be 111111. The new buckets you obtain by substituting in ??1100
 * with either 0 or 1 can be targeted only by keys we already visited
 * when scanning the bucket 1100 in the smaller hash table.
 *
 * By iterating the higher bits first, because of the inverted counter, the
 * cursor does not need to restart if the table size gets bigger. It will
 * continue iterating using cursors without '1100' at the end, and also
 * without any other combination of the final 4 bits already explored.
 *
 * Similarly when the table size shrinks over time, for example going from
 * 16 to 8, if a combination of the lower three bits (the mask for size 8
 * is 111) were already completely explored, it would not be visited again
 * because we are sure we tried, for example, both 0111 and 1111 (all the
 * variations of the higher bit) so we don't need to test it again.
 *
 * WAIT... YOU HAVE *TWO* TABLES DURING REHASHING!
 *
 * Yes, this is true, but we always iterate the smaller table first, then
 * we test all the expansions of the current cursor into the larger
 * table. For example if the current cursor is 101 and we also have a
 * larger table of size 16, we also test (0)101 and (1)101 inside the larger
 * table. This reduces the problem back to having only one table, where
 * the larger one, if it exists, is just an expansion of the smaller one.
 *
 * LIMITATIONS
 *
 * This iterator is completely stateless, and this is a huge advantage,
 * including no additional memory used.
 *
 * The disadvantages resulting from this design are:
 *
 * 1) It is possible we return elements more than once. However this is usually
 *    easy to deal with in the application level.
 * 2) The iterator must return multiple elements per call, as it needs to always
 *    return all the keys chained in a given bucket, and all the expansions, so
 *    we are sure we don't miss keys moving during rehashing.
 * 3) The reverse cursor is somewhat hard to understand at first, but this
 *    comment is supposed to help.
 */
unsigned long dictScan(dict *d,
                       unsigned long v,
                       dictScanFunction *fn,
                       dictScanBucketFunction* bucketfn,
                       void *privdata)
{
    dictht *t0, *t1;
    const dictEntry *de, *next;
    unsigned long m0, m1;

    if (dictSize(d) == 0) return 0;

    /* This is needed in case the scan callback tries to do dictFind or alike. */
    dictPauseRehashing(d);

    if (!dictIsRehashing(d)) {
        t0 = &(d->ht[0]);
        m0 = t0->sizemask;

        /* Emit entries at cursor */
        if (bucketfn) bucketfn(privdata, &t0->table[v & m0]);
        de = t0->table[v & m0];
        while (de) {
            next = de->next;
            fn(privdata, de);
            de = next;
        }

        /* Set unmasked bits so incrementing the reversed cursor
         * operates on the masked bits */
        v |= ~m0;

        /* Increment the reverse cursor */
        v = rev(v);
        v++;
        v = rev(v);

    } else {
        t0 = &d->ht[0];
        t1 = &d->ht[1];

        /* Make sure t0 is the smaller and t1 is the bigger table */
        if (t0->size > t1->size) {
            t0 = &d->ht[1];
            t1 = &d->ht[0];
        }

        m0 = t0->sizemask;
        m1 = t1->sizemask;

        /* Emit entries at cursor */
        if (bucketfn) bucketfn(privdata, &t0->table[v & m0]);
        de = t0->table[v & m0];
        while (de) {
            next = de->next;
            fn(privdata, de);
            de = next;
        }

        /* Iterate over indices in larger table that are the expansion
         * of the index pointed to by the cursor in the smaller table */
        do {
            /* Emit entries at cursor */
            if (bucketfn) bucketfn(privdata, &t1->table[v & m1]);
            de = t1->table[v & m1];
            while (de) {
                next = de->next;
                fn(privdata, de);
                de = next;
            }

            /* Increment the reverse cursor not covered by the smaller mask.*/
            v |= ~m1;
            v = rev(v);
            v++;
            v = rev(v);

            /* Continue while bits covered by mask difference is non-zero */
        } while (v & (m0 ^ m1));
    }

    dictResumeRehashing(d);

    return v;
}

/* ------------------------- private functions ------------------------------ */

/* Because we may need to allocate huge memory chunk at once when dict
 * expands, we will check this allocation is allowed or not if the dict
 * type has expandAllowed member function. */
static int dictTypeExpandAllowed(dict *d) {
    if (d->type->expandAllowed == NULL) return 1;
    return d->type->expandAllowed(
                    _dictNextPower(d->ht[0].used + 1) * sizeof(dictEntry*),
                    (double)d->ht[0].used / d->ht[0].size);
}

/*
 * 根据需要，初始化字典（的哈希表），或者对字典（的现有哈希表）进行扩展
 */
static int _dictExpandIfNeeded(dict *d)
{
    // 渐进式 rehash 已经在进行了，不能扩展和初始化 直接返回
    if (dictIsRehashing(d)) return DICT_OK;

    // 如果字典（的 0 号哈希表）为空，那么创建并返回初始化大小的 0 号哈希表
    if (d->ht[0].size == 0) return dictExpand(d, DICT_HT_INITIAL_SIZE);

    /* 一下两个条件之一为真时，对字典进行扩展
       1）字典已使用节点数和字典大小之间的比率接近 1：1，并且 dict_can_resize 为真
       2）已使用节点数和字典大小之间的比率超过 dict_force_resize_ratio
    */
    if (d->ht[0].used >= d->ht[0].size &&
        (dict_can_resize ||
        // 新哈希表的大小至少是目前已使用节点数的两倍
         d->ht[0].used/d->ht[0].size > dict_force_resize_ratio) &&
        dictTypeExpandAllowed(d))
    {
        // 满足扩容的话，就会调用这个进行扩容
        return dictExpand(d, d->ht[0].used + 1);
    }
    return DICT_OK;
}

/**
 * 计算第一个大于等于 size 的 2 的 N 次方，用作新哈希表大小的值
 * 根据ht[0]的大小，确定rehash操作需要的ht[1]的大小
 */
static unsigned long _dictNextPower(unsigned long size)
{
    unsigned long i = DICT_HT_INITIAL_SIZE;

    if (size >= LONG_MAX) return LONG_MAX + 1LU;
    while(1) {
        if (i >= size)
            return i;
        i *= 2;
    }
}

/*
 * 返回可以将 key 插入到哈希表的索引位置
 * 如果 key 已经存在于哈希表，那么返回 -1
 *
 * 注意，如果字典正在进行 rehash ，那么总是返回 1 号哈希表的索引。
 * 因为在字典进行 rehash 时，新节点总是插入到 1 号哈希表。
 */
static long _dictKeyIndex(dict *d, const void *key, uint64_t hash, dictEntry **existing)
{
    unsigned long idx, table;
    //哈希结点
    dictEntry *he;
    if (existing) *existing = NULL;

    //如果需要的话，扩展哈希表
    if (_dictExpandIfNeeded(d) == DICT_ERR)
        return -1;
    for (table = 0; table <= 1; table++) {
        idx = hash & d->ht[table].sizemask;
        // 获得哈希表索引上的结点（存在哈希冲突的话，改结点就是链表结点）
        he = d->ht[table].table[idx];
        while(he) {
            if (key==he->key || dictCompareKeys(d, key, he->key)) {
                if (existing) *existing = he;
                return -1;
            }
            // 实际这里使用了拉链法来解决冲突
            he = he->next;
        }
        // 如果运行到这里时，说明 0 号哈希表中所有节点都不包含 key
        // 如果这个时候没有进行rehash，则跳出去，否则在ht[1]继续计算索引位置
        if (!dictIsRehashing(d)) break;
    }
    return idx;
}

void dictEmpty(dict *d, void(callback)(void*)) {
    _dictClear(d,&d->ht[0],callback);
    _dictClear(d,&d->ht[1],callback);
    d->rehashidx = -1;
    d->pauserehash = 0;
}

void dictEnableResize(void) {
    dict_can_resize = 1;
}

void dictDisableResize(void) {
    dict_can_resize = 0;
}

uint64_t dictGetHash(dict *d, const void *key) {
    return dictHashKey(d, key);
}

/* Finds the dictEntry reference by using pointer and pre-calculated hash.
 * oldkey is a dead pointer and should not be accessed.
 * the hash value should be provided using dictGetHash.
 * no string / key comparison is performed.
 * return value is the reference to the dictEntry if found, or NULL if not found. */
dictEntry **dictFindEntryRefByPtrAndHash(dict *d, const void *oldptr, uint64_t hash) {
    dictEntry *he, **heref;
    unsigned long idx, table;

    if (dictSize(d) == 0) return NULL; /* dict is empty */
    for (table = 0; table <= 1; table++) {
        idx = hash & d->ht[table].sizemask;
        heref = &d->ht[table].table[idx];
        he = *heref;
        while(he) {
            if (oldptr==he->key)
                return heref;
            heref = &he->next;
            he = *heref;
        }
        if (!dictIsRehashing(d)) return NULL;
    }
    return NULL;
}

/* ------------------------------- Debugging ---------------------------------*/

#define DICT_STATS_VECTLEN 50
size_t _dictGetStatsHt(char *buf, size_t bufsize, dictht *ht, int tableid) {
    unsigned long i, slots = 0, chainlen, maxchainlen = 0;
    unsigned long totchainlen = 0;
    unsigned long clvector[DICT_STATS_VECTLEN];
    size_t l = 0;

    if (ht->used == 0) {
        return snprintf(buf,bufsize,
            "No stats available for empty dictionaries\n");
    }

    /* Compute stats. */
    for (i = 0; i < DICT_STATS_VECTLEN; i++) clvector[i] = 0;
    for (i = 0; i < ht->size; i++) {
        dictEntry *he;

        if (ht->table[i] == NULL) {
            clvector[0]++;
            continue;
        }
        slots++;
        /* For each hash entry on this slot... */
        chainlen = 0;
        he = ht->table[i];
        while(he) {
            chainlen++;
            he = he->next;
        }
        clvector[(chainlen < DICT_STATS_VECTLEN) ? chainlen : (DICT_STATS_VECTLEN-1)]++;
        if (chainlen > maxchainlen) maxchainlen = chainlen;
        totchainlen += chainlen;
    }

    /* Generate human readable stats. */
    l += snprintf(buf+l,bufsize-l,
        "Hash table %d stats (%s):\n"
        " table size: %lu\n"
        " number of elements: %lu\n"
        " different slots: %lu\n"
        " max chain length: %lu\n"
        " avg chain length (counted): %.02f\n"
        " avg chain length (computed): %.02f\n"
        " Chain length distribution:\n",
        tableid, (tableid == 0) ? "main hash table" : "rehashing target",
        ht->size, ht->used, slots, maxchainlen,
        (float)totchainlen/slots, (float)ht->used/slots);

    for (i = 0; i < DICT_STATS_VECTLEN-1; i++) {
        if (clvector[i] == 0) continue;
        if (l >= bufsize) break;
        l += snprintf(buf+l,bufsize-l,
            "   %s%ld: %ld (%.02f%%)\n",
            (i == DICT_STATS_VECTLEN-1)?">= ":"",
            i, clvector[i], ((float)clvector[i]/ht->size)*100);
    }

    /* Unlike snprintf(), return the number of characters actually written. */
    if (bufsize) buf[bufsize-1] = '\0';
    return strlen(buf);
}

void dictGetStats(char *buf, size_t bufsize, dict *d) {
    size_t l;
    char *orig_buf = buf;
    size_t orig_bufsize = bufsize;

    l = _dictGetStatsHt(buf,bufsize,&d->ht[0],0);
    buf += l;
    bufsize -= l;
    if (dictIsRehashing(d) && bufsize > 0) {
        _dictGetStatsHt(buf,bufsize,&d->ht[1],1);
    }
    /* Make sure there is a NULL term at the end. */
    if (orig_bufsize) orig_buf[orig_bufsize-1] = '\0';
}

/* ------------------------------- Benchmark ---------------------------------*/

#ifdef REDIS_TEST

uint64_t hashCallback(const void *key) {
    return dictGenHashFunction((unsigned char*)key, strlen((char*)key));
}

int compareCallback(void *privdata, const void *key1, const void *key2) {
    int l1,l2;
    DICT_NOTUSED(privdata);

    l1 = strlen((char*)key1);
    l2 = strlen((char*)key2);
    if (l1 != l2) return 0;
    return memcmp(key1, key2, l1) == 0;
}

void freeCallback(void *privdata, void *val) {
    DICT_NOTUSED(privdata);

    zfree(val);
}

char *stringFromLongLong(long long value) {
    char buf[32];
    int len;
    char *s;

    len = sprintf(buf,"%lld",value);
    s = zmalloc(len+1);
    memcpy(s, buf, len);
    s[len] = '\0';
    return s;
}

dictType BenchmarkDictType = {
    hashCallback,
    NULL,
    NULL,
    compareCallback,
    freeCallback,
    NULL,
    NULL
};

#define start_benchmark() start = timeInMilliseconds()
#define end_benchmark(msg) do { \
    elapsed = timeInMilliseconds()-start; \
    printf(msg ": %ld items in %lld ms\n", count, elapsed); \
} while(0)

/* ./redis-server test dict [<count> | --accurate] */
int dictTest(int argc, char **argv, int accurate) {
    long j;
    long long start, elapsed;
    dict *dict = dictCreate(&BenchmarkDictType,NULL);
    long count = 0;

    if (argc == 4) {
        if (accurate) {
            count = 5000000;
        } else {
            count = strtol(argv[3],NULL,10);
        }
    } else {
        count = 5000;
    }

    start_benchmark();
    for (j = 0; j < count; j++) {
        int retval = dictAdd(dict,stringFromLongLong(j),(void*)j);
        assert(retval == DICT_OK);
    }
    end_benchmark("Inserting");
    assert((long)dictSize(dict) == count);

    /* Wait for rehashing. */
    while (dictIsRehashing(dict)) {
        dictRehashMilliseconds(dict,100);
    }

    start_benchmark();
    for (j = 0; j < count; j++) {
        char *key = stringFromLongLong(j);
        dictEntry *de = dictFind(dict,key);
        assert(de != NULL);
        zfree(key);
    }
    end_benchmark("Linear access of existing elements");

    start_benchmark();
    for (j = 0; j < count; j++) {
        char *key = stringFromLongLong(j);
        dictEntry *de = dictFind(dict,key);
        assert(de != NULL);
        zfree(key);
    }
    end_benchmark("Linear access of existing elements (2nd round)");

    start_benchmark();
    for (j = 0; j < count; j++) {
        char *key = stringFromLongLong(rand() % count);
        dictEntry *de = dictFind(dict,key);
        assert(de != NULL);
        zfree(key);
    }
    end_benchmark("Random access of existing elements");

    start_benchmark();
    for (j = 0; j < count; j++) {
        dictEntry *de = dictGetRandomKey(dict);
        assert(de != NULL);
    }
    end_benchmark("Accessing random keys");

    start_benchmark();
    for (j = 0; j < count; j++) {
        char *key = stringFromLongLong(rand() % count);
        key[0] = 'X';
        dictEntry *de = dictFind(dict,key);
        assert(de == NULL);
        zfree(key);
    }
    end_benchmark("Accessing missing");

    start_benchmark();
    for (j = 0; j < count; j++) {
        char *key = stringFromLongLong(j);
        int retval = dictDelete(dict,key);
        assert(retval == DICT_OK);
        key[0] += 17; /* Change first number to letter. */
        retval = dictAdd(dict,key,(void*)j);
        assert(retval == DICT_OK);
    }
    end_benchmark("Removing and adding");
    dictRelease(dict);
    return 0;
}
#endif
